Preparation of an aqueous polymer dispersion by the free radical aqueous emulsion polymerization method

ABSTRACT

In a process for the preparation of an aqueous polymer dispersion by the free radical aqueous emulsion polymerization method, the polymerization is carried out in the presence of frozen micelles of amphiphilic substances.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the preparation of anaqueous polymer dispersion by polymerizing monomers having at least onevinyl group by the free radical aqueous emulsion polymerization method,in which an amphiphilic substance is added to the polymerization vesselbefore and/or during the polymerization.

2. Description of the Background

Aqueous polymer dispersions (latices) are generally known. They arefluid systems which contain, as the disperse phase in an aqueousdispersing medium, polymer coils (i.e. polymer particles) consisting ofa plurality of intertwined polymer chains.

The diameter of the polymer particles is frequently from 10 to 2000 nm.

As in the case of polymer solutions on evaporation of the solvent,aqueous polymer dispersions have the potential to form polymer films onevaporation of the aqueous dispersing medium, and they are thereforeused in particular as binders, for example for paints or for materialsfor coating leather, paper or plastics films. They are becomingincreasingly important owing to their environmentally friendlyproperties.

An important feature of aqueous polymer dispersions is the diameter ofthe polymer particles present as the disperse phase, since the size ofthe polymer particles or their size distribution plays a role indetermining a number of performance characteristics of aqueous polymerdispersions. For example, films of finely divided aqueous polymerdispersions have high gloss (cf. for example, Progress in OrganicCoatings 6 (1978), 22). Furthermore, the power of finely divided aqueouspolymer dispersions to penetrate into porous but relatively densesubstrates, such as paper, leather or a render surface is greater thanthat of coarse-particled aqueous polymer dispersions (for example,Dispersionen synth. Hochpolymerer, Part II, Anwendung, H. Reinhard,Springer-Verlag, Berlin (1969), page 4).

On the other hand, coarse-particled aqueous polymer dispersions have,for example, lower flow resistance than finely divided aqueous polymerdispersions, the composition and solids concentration otherwise beingidentical (for example, Dispersionen synth. Hochpolymerer, Teil II,Anwendung, H. Reinhard, Springer-Verlag, Berlin (1969), page 5). Aqueouspolymer dispersions whose polymer particle diameters are distributedover a relatively large diameter range also have advantageous flowbehavior (cf. for example, DE-A 42 13 965).

Establishing the diameters of the dispersed polymer particles in acontrolled, reproducible manner tailored to the particular intended useis therefore of key importance in the preparation of an aqueous polymerdispersion.

The most important method for the preparation of aqueous polymerdispersions is the free radical emulsion polymerization method, inparticular the free radical aqueous emulsion polymerization method.

In the latter method, monomers having at least one vinyl group areusually subjected to free radical polymerization under the action offree radical polymerization initiators dissolved in the aqueous medium,to give polymer particles present directly as the disperse phase in theaqueous dispersing medium. The aqueous polymer dispersions prepared bythe free radical aqueous emulsion polymerization method are usuallyreferred to as aqueous primary dispersions, in order to distinguish themfrom the aqueous secondary dispersions. In the case of the latter, thepolymerization is carried out in a nonaqueous medium. Dispersing in theaqueous medium is not effected until after the polymerization reactionis complete.

The monomers to be polymerized are distributed in the form of droplets(the droplet diameter is frequently from 2 to 10 μm) in the aqueousmedium with formation of an aqueous monomer emulsion. However, thesemonomer droplets are not the sites of the polymerization but act merelyas a monomer reservoir. Rather, the polymerization sites are formed inthe aqueous phase, which always contains a limited amount of themonomers to be polymerized and the free radical polymerization initiatorin dissolved form. Chemical reaction of these reactants present insolution results in the formation of oligomer radicals, which areprecipitated as primary particles above a critical chain length(homogeneous nucleation). The formation of primary particles presumablytakes place up to the point at which the rate of formation of the freeradicals in the aqueous phase is equal to the rate of theirdisappearance due to free radical capture by polymer particles alreadyformed. This polymer particle formation phase is then followed by thepolymer particle growth phase, i.e. the monomers to be polymerizeddiffuse from the monomer droplets acting as a reservoir, via the aqueousphase, to the primary particles formed (whose number and surface areaare very much greater than those of the monomer droplets), in order tobe incorporated into said primary particles by polymerization (cf. forexample, Faserforschung und Textiltechnik 28 (1977), Part 7, Zeitschriftfür Polymerforschung, page 309). By controlled addition of suitabledispersants, both the disperse phase of the monomer droplets and thedisperse phase of the polymer particles formed are, if required,stabilized.

While the process of polymer particle growth usually takes placesystematically, the polymer particle formation is essentially astochastic process, i.e. the number of primary polymer particles formedand hence the diameters of the final polymer particles resulting afterthe end of the polymerization fluctuate from polymerization batch topolymerization batch. The product quality fluctuates in a correspondingmanner (identical reproduction is usually not possible). This appliesvery particularly in the case of a high solids volume content (≧50Vol.-%) of the aqueous polymer dispersion, since, for example, theviscosity of highly concentrated aqueous polymer dispersions isparticularly sensitive to the number and size of the polymer particlescontained in dispersed form.

It is known that a controlled free radical aqueous emulsionpolymerization procedure is possible by initiating it in the presence ofa surfactant dissolved in the aqueous medium, the surfactant content ofthe aqueous medium being such that it is above the critical micelleformation concentration of said medium (cf. for example, High Polymers,Vol. IX, Emulsion Polymerization, Interscience Publishers, Inc., NewYork, Third Printing, 1965, page 1 et seq.).

The term surfactant means amphiphilic substances which, on dissolutionin water, are capable of reducing the surface tension a of pure watersignificantly (as a rule by at least 25%, based on the a value of purewater) before reaching the critical micelle formation concentration.

The term “amphiphilic” indicates that surfactants have both hydrophilicand hydrophobic groups. Hydrophilic groups are those which are drawninto the aqueous phase, whereas hydrophobic groups are forced out of theaqueous phase.

In highly dilute aqueous solutions, surfactants are therefore presentessentially independent molecules in solution, their amphiphilicstructure resulting in accumulation at the water surface with orientedadsorption, which reduces the surface tension.

In concentrated aqueous solutions, on the other hand, surfactants arepresent predominantly as micelles in solution, i.e. the surfactantmolecules are arranged in the aqueous solution predominantly in a stateof relatively high aggregation, i.e. as micelles, in which they areoriented in such a way that the hydrophilic groups face the aqueousphase and the hydrophobic groups point toward the interior of themicelle. As the surfactant concentration increases further, essentiallyonly the number of micelles per unit volume increases, but not thenumber of surfactant molecules dissolved in molecular form per unitvolume.

The transition from the aqueous molecular solution to the aqueousmicellar solution usually takes place relatively abruptly, as a functionof the surfactant concentration, which is evident from correspondinglyabrupt changes in the concentration dependence of many macroscopicproperties (for example the surface tension) and defines the criticalmicelle formation concentration (usually stated as molar concentrationc.m.c.) (inflexional point in the concentration dependence of theproperty). At concentrations above the critical micelle formationconcentration, the term micellar solutions is used. Here, the termsolution is intended to express the fact that the visual appearance of amicellar aqueous surfactant solution, like that of a molecular aqueoussurfactant solution, is the same as that of a clear aqueous solution.The relative molecular weight of surfactants is usually <2000, and thereis usually a rapid exchange (a dynamic equilibrium) in their micellaraqueous solutions between the various surfactant fractions present insolution in molecular and micellar form.

The following are typical examples of surfactants (source: Ull-mannsEncyclopädie der technischen Chemie, Verlag Chemie, 4th edition, Vol.22, page 456 et seq.):

a) Perfluorononanecarboxylic acid (c.m.c. at 20° C. and 1 atm inwater=10⁻⁵ mol/l; σ of the substituted aqueous solution=20 mN/m);

b) Sodium 1-decyl sulfate (c.m.c. at 20° C. and 1 atm in water=3.4·10⁻²mol/l; σ of the associated aqueous solution=40 mN/m).

The surface tension of pure water at 20° C. and 1 atm is 73 mN/m.

It is now generally assumed that the surfactant micelles present in anaqueous medium are nucleation centres for the formation of primarypolymer particles (the term micellar nucleation is also used). If thefree radical aqueous emulsion polymerization is initiated, for example,in the presence of a large number of surfactant micelles, many smallfinal polymer particles are obtained, whereas initiation in the presenceof a small number of surfactant micelles gives a few large polymerparticles. At the same time, the surfactant generally reduces both thepolymer particle/aqueous medium interfacial tension and the monomerdroplet/aqueous medium interfacial tension and is thus capable ofstabilizing the particular disperse phase as a dispersant, which has anadvantageous effect on the free radical aqueous emulsion polymerization.On the other hand, the decrease in surface tension is usuallydisadvantageous, said decrease being caused by the surfactant andincreasing the tendency to foam formation.

While the validity of the abovementioned relationships may besatisfactory qualitatively (smaller polymer particles are obtained withincreasing amount of surfactant, and vice versa; cf. Dispersionensynthetischer Hochpolymerer, Part I, F. Holscher, Springer-Verlag,Berlin (1969), page 81), the quantitative relationship is as a rule justas unsatisfactory as the reproducibility.

EP-B 40 419 (e.g. page 5, line 16 et seq. and Example 1), DE-A 23 21 835(e.g. page 14, line 9 et seq.) and Encyclopedia of Polymer Science andTechnology, Vol. 5, John Wiley & Sons Inc., New York (1966), page 847,therefore recommend that, for establishing the final polymer particlesize in a controlled manner, the polymer particle formation phase shouldbe separated from the polymer particle growth, i.e. a defined amount ofa separately preformed aqueous polymer dispersion (nuclear or seedpolymer dispersion) is added, for example before the beginning of thefree radical aqueous emulsion polymerization, and the polymer particlescontained in this seed are allowed only to grow in the course of theactual free radical aqueous emulsion polymerization. The diameter of theseed polymer particles and the ratio of initially taken seed polymerparticles and monomers to be polymerized essentially determine the sizeof the final polymer particles in the resulting aqueous polymerdispersion. The more finely divided the seed and the greater the amountof seed used, the smaller are the resulting final polymer particles fora given amount of monomer. If a broad distribution of the diameters ofthe polymer particles is desired, additional seed polymer dispersion isadded to the polymerization vessel also during the free radical aqueousemulsion polymerization of the monomers. In this way, the resultingaqueous polymer dispersion comprises various generations of seed polymerparticles grown to different final sizes. A similar effect can also beobtained by initiating the formation of new micelles in the course ofthe free radical aqueous emulsion polymerization of the monomers byadding a larger amount of surfactant.

However, the disadvantage of the free radical aqueous emulsionpolymerization method with the addition of an aqueous seed polymerdispersion is that the aqueous seed polymer dispersion has to be storedprior to its use, which frequently entails problems owing to the basicsensitivity of aqueous polymer dispersions (the subsequent attempt toreduce their interface) to frost, shearing, superficial drying andvibration. Moreover, identical product preparation at differentproduction locations requires the corresponding identical availabilityof such an aqueous seed polymer dispersion. An additional problem is theproduction of an aqueous seed polymer dispersion in a reproduciblemanner.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for thepreparation of an aqueous polymer dispersion by polymerizing monomershaving at least one vinyl group by the free radical aqueous emulsionpolymerization method, which process permits the free radical aqueousemulsion polymerization to be carried out in a controlled mannerqualitatively comparable with the seed procedure, but does not have thedisadvantages of seed storage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a graphical plot of surface tension versusconcentration for micellar aqueous solutions of the sodium salts of thetwo-block copolymers of the present invention.

FIG. 2 illustrates a graphical plot of (1/d³ ₅₀)·10⁷ versus amount ofBP1 added to the polymerization.

FIG. 3 illustrates a graphical plot of (1/d³ ₅₀)·10⁷ versus amount ofBP8 added to the polymerization.

FIG. 4B illustrates the determination of apparent weight averagemolecular weight of units migrating in a centrifugal field as a functionof the solution concentration from the exparential concentration curvesof sedimentation-diffusion equilibrium runs.

FIG. 4B illustrates a plot of reciprocal values as a function ofconcentration to give a relative M_(w) of the migrating unit of600000±25% on linear extrapolation to the concentration 0.

FIG. 5 illustrates a plot of the inverse values of scattering intensitydifferences as a function of the concentration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

We have found that this object is achieved by a process for thepreparation of an aqueous polymer dispersion by polymerizing monomershaving at least one vinyl group by the free radical aqueous emulsionpolymerization method, in which an amphiphilic substance is added to thepolymerization vessel before and/or during the polymerization, wherein

1 l of water at 20° C. and 1 atm is capable of taking up at least 10⁻⁴mol of the amphiphilic substance in micellar solution;

the critical micelle formation concentration of the amphiphilicsubstance at 20° C. and 1 atm in water is <10⁻⁶ mol/l and

the surface tension a of an aqueous molecular and/or micellar solutionof the amphiphilic substance in the molar concentration range(0<C_(M)<10⁻⁴)mol/l at 20° C. and 1 atm does not fall below 60 mN/m.

The c.m.c. of the amphiphilic substance to be used according to theinvention, at 20° C. and 1 atm in water, is preferably ≦10^(−6.25),preferably ≦10^(−6.5), particularly preferably ≦10^(−6.75), morepreferably ≦10^(−7.0), very particularly preferably ≦10^(−7.25), evenmore preferably ≦10^(−7.5), mol/l.

It is also advantageous if the surface tension σ of an aqueous molecularand/or micellar solution of the amphiphilic substance to be usedaccording to the invention, in the molar concentration range(0<C_(M)≦10⁻⁴)mol/l, at 20° C. and 1 atm, does not fall below 62.5,preferably 65, particularly preferably 67,5, very particularlypreferably 70, even more preferably 71.5, mN/m. Preferably, the surfacetension does not fall below the abovementioned values even when a molarconcentration of 10⁻³ mol/l is reached.

It is furthermore advantageous if the amphiphilic substance to be usedaccording to the invention is such that 1 l of water at 20° C. and 1 atmis capable of taking up at least 10⁻³, preferably at least 10⁻²,particularly preferably at least 10⁻¹, mol or 1 mol of the amphiphilicsubstance in micellar solution.

The present invention is based on the observation that not onlysurfactants, i.e. lower molecular weight amphiphilic substances whichare capable of significantly reducing the surface tension of water, arecapable of forming micellar aqueous solutions above a c.m.c., but thatthis property can also be attributed essentially very generally toamphiphilic substances.

The c.m.c. is essentially determined by the type of hydrophobic groupsof the amphiphilic substance, within a homologous series the c.m.c.assuming lower values with increasing extent (e.g. increasing length(number of carbon atoms) of the alkyl group) of the hydrophobic group.The effect of the hydrophilic group on the c.m.c. is accordinglycomparatively small.

At the same time, the rate of exchange between the different fractionsdissolved in molecular or micellar form in aqueous micellar solutions ofamphiphilic substances decreases with increasing length of thehydrophobic group, which may result in the dynamic equilibrium statebetween the different fractions being reached only after a relativelylong time or only at elevated temperatures. As a rule, this isaccompanied by a decrease in the fraction dissolved in molecular form inthe equilibrium state, which is evident from a lesser ability to reducethe surface tension.

The possible explanation for the abovementioned observations is that theincrease in the attractive interaction of the hydrophobic groups withone another as their length increases is greater than the increase intheir attractive interaction with the aqueous phase, and it is for thisreason that, with increasing length, they are increasingly displacedfrom the aqueous phase and forced into aggregation (micelle formation)(with increasing residence time therein). The type of hydrophilic groupessentially decides only whether the aggregate can be held in solutionor not.

In other words, the amphiphilic substances to be added according to theinvention are those whose fraction dissolved in molecular form in theirmicellar aqueous solution is particularly small and in which theexchange between the various fractions dissolved in micellar andmolecular form in their micellar aqueous solutions takes placeparticularly slowly (increased kinetic stability).

Our investigations have shown that micellar aqueous solutions of theamphiphilic substances to be added according to the invention areparticularly suitable as nucleation centres of the controlled freeradical aqueous emulsion polymerization of monomers having at least onevinyl group.

The difference compared with the micellar aqueous solutions of theclassical surfactants is presumably the following. If the formation ofpolymer particles is initiated in some micelles at the beginning of thefree radical aqueous emulsion polymerization in the presence ofclassical aqueous micellar systems, other micelles not yet initiatedbegin to disintegrate (owing to the rapid exchange) in order to supportthe stabilization of the surface of the growing polymer particles, withthe result that the quantitative relationship between the number ofmicelles originally present and the number of polymer particles formedis lost.

With increasing kinetic stability of the micelles, the phenomenon ofmicellar disintegration decreases in the course of the formation of thepolymer particles, and the polymer particle formation phase increasinglyapproaches the limiting case where each micelle originally presentbecomes the nucleation centre of a polymer particle (as a rule, thestability of the micelles to the addition of foreign electrolyte alsoincreases with increasing kinetic stability of the micelles). In otherwords, as the kinetic micellar stability increases, an aqueous micellarsolution becomes increasingly capable of affecting the course of freeradical aqueous emulsion polymerization in the same way as an aqueousseed polymer dispersion. In contrast to the latter, however, it can beproduced in a reproducible manner and stored in the dry state, whichavoids the disadvantages of storing a seed polymer dispersion.

In the simplest case, the desired micellar effect of the amphiphilicsubstance to be added according to the invention can be produced in thecourse of free radical aqueous emulsion polymerization by adding theamphiphilic substance to the aqueous polymerization medium as such in anamount above the c.m.c. As a rule, however, the amphiphilic substance tobe added according to the invention is added in the form of a preformedmicellar solution (suitable solvents are both water and a water-miscibleorganic solvent or a mixture of water and such a solvent).

This applies in particular when the amphiphilic substance is not capableof directly forming an aqueous micellar solution with water understandard temperature and pressure conditions (20° C., 1 atm). In thesecases, it is frequently possible to prepare a micellar aqueous solutionby first dissolving the amphiphilic substance to be added according tothe invention in a water-miscible organic solvent or in a mixture ofwater and such an organic solvent to give a molecular and/or micellarsolution (for example, in dioxane, tetrahydrofuran or mixtures thereofwith water) and then converting this molecular and/or micellar solution(which, according to the invention, can also frequently be addeddirectly to the aqueous polymerization medium), for example by means ofdialysis or repeated additions of small amounts of water and subsequentremoval of the organic solvent used by distillation, into an aqueousmicellar solution (instead of water, an aqueous solution of an acidand/or base is also frequently used) and, if required, concentratingsaid solution by evaporating water. Such aqueous micellar solutionsproduced at 1 atm and 20° C. are as a rule not in thermodynamicequilibrium. However, they usually have higher kinetic stability, i.e.the micelles contained therein in dissolved form behave likequasi-molecular structures between which there is virtually no moreexchange. The average residence time of an amphiphilic molecule in suchmicelles may be several hours or days, even at elevated temperatures. Towhat extent such aqueous micellar solutions have a c.m.c. at all isoften unclear. If they do, it is at very low concentrations. Suchaqueous micellar solutions which can normally be produced only by anindirect method and are not in thermodynamic equilibrium are frequentlyreferred to in the literature as solutions of frozen micelles (cf. forexample, Polymer Preprints 32 (1) (1991), 525, Makromol. Chem. Macromol.Symp. 58 (1992), 195-199, or Langmuir 9 (1993), 1741-1748). As a rule,they do not change their macroscopic appearance over several days (at20° C. and 1 atm). Aqueous micellar solutions of corresponding kineticstability are rather seldom obtainable by directly dissolvingamphiphilic substances.

In particular, aqueous solutions of frozen micelles of amphiphilicsubstances to be added according to the invention can be used instead ofaqueous seed polymer dispersions for controlled free radical aqueousemulsion polymerizations (they may be added as such or, under certaincircumstances, also produced in situ in the aqueous polymerizationmedium by adding, for example, a solution of the amphiphilic substanceto be added according to the invention in a water-miscible organicsolvent to the aqueous polymerization medium). By diluting aqueoussolutions of frozen micelles, it is possible, owing to their kineticstability, to produce aqueous micellar solutions whose concentration ofamphiphilic substance is below their c.m.c. and to use these solutionsaccording to the invention.

The surface tension of the aqueous solutions is as a rule not a suitablemeasured quantity for determining the c.m.c., at very low concentrationsin aqueous solution, of the amphiphilic substances to be added accordingto the invention.

The c.m.c. data used in this publication therefore relate toinvestigations of the concentration dependence of the scatteringbehavior (Classical Light Scattering From Polymer Solutions, PavelKratochvil, Elsevier, New York (1987), in particular Section 2.1.2) ofthe relevant aqueous solutions (classical light scattering; Price, C.,Pure Appl. Chem. 55 (1983), 1563; Price, C.; Chan, E. K. M.;Stubbersfield, R. B., Eur. Polym. J. 23 (1987), 649, and Price, C.;Stubbersfield, R. B.; El-Kafrawy, S.; Kendall, K. D., Br. Polym. J. 21(1989), 391) or, if the sensitivity of this method of investigation isinsufficient, to investigations of the fluorescence behavior ofhydrophobic dyes, such as Fluorol® 7GA or pyrene, which accumulate inthe hydrophobic inner region of the micelles and alter theirfluorescence behavior (Zhao, C. L.; Winnik, M. A.; Riess, G.; Croucher,M. D., Langmuir 6 (1990), 514; Wilhelm, M.; Zhao, C. L.; Wang, Y.; Xu,R.; Winnik, M. A., Macromolecules 24 (1991), 1033; Astafieva, I.; Zhong,X. F.; Eisenberg, A., Macromolecules 26 (1993), 7339, and Astafieva, I.;Khongaz, K.; Eisenberg, A., Macromolecules 28 (1995), 7127).

Block polymers in which at least one of the blocks present (hydrophilicgroup) is readily water-soluble as an independent polymer and at leastone other of the blocks present (hydrophobic group) is only slightlywater-soluble as an independent polymer form a particularly clear typeof amphiphilic substances.

The statements made so far are therefore illuminating in a particularlyevident way on the basis of a closer consideration of such amphiphilicblock polymers, in particular in the case of the two-block polymers.

The term block polymer refers to polymers whose molecules (instead ofthe term molecule, the literature also frequently uses the more generalterm unimer in connection with micelle-forming amphiphilic substances,in order to distinguish the individual species from their micellaraggregation, since strictly the term molecule does not include, forexample, polyelectrolyte types) consist of preferably linear, linkedblocks, the blocks being bonded to one another directly or byconstitutional units which are not part of the blocks, and the termblock meaning a segment of a polymer molecule (unimer) which comprises aplurality of identical constitutional units and has at least oneconstitutional or configurative feature which does not occur in thedirectly adjacent segments. Two-block polymers accordingly consist oftwo blocks.

Block polymers are obtainable in a simple manner by first linking onetype of monomeric building blocks successively in series, thencontinuing this linkage with another type of monomeric building blocks,subsequently carrying out further changes of the monomeric buildingblock type if necessary, and thus producing two- or three-block polymersor polymers having a larger number of blocks as required. Starting frommonomeric building blocks having at least one vinyl group, the linkagecan be effected both within the individual block and between the blocks,for example in a manner known per se by initiated polymerization(keyword:

living polymers; cf. for example, Ullmanns Encyklopädie der technischenChemie, Vol. 13, 4th edition, Verlag Chemie, New York, page 599). Theinitiated polymerization is designed in a manner known per se so that,after one monomer type has been completely consumed, macroinitiatorswhich are still active or can be reactivated by suitable measures areobtained and continue to grow after the addition of the next monomertype, until their activity is deliberately terminated by the addition ofsuitable inhibitors.

A particularly frequently used method for such initiated sequentialpolymerization is sequential anionic polymerization (cf. for example,U.S. Pat. No. 3,251,905; U.S. Pat. No. 3,390,207; U.S. Pat. No.3,598,887; U.S. Pat. No. 4,219,627; Macromolecules 27 (1994), 4908;Polymer, 32 Number 12 (1991), 2279; Macromolecules 27 (1994), 4615;Macromolecules 27 (1994), 4635, and Macromolecules 24 (1991), 4997). Inthe same way as the free radical polymerization, it takes placeaccording to a chain reaction scheme. However, it is not an initiatorradical which functions as the initiator but an initiator anion, whichdonates its charge to the growing macromolecule, which in turn is thuscapable of acting as an initiator anion and continuing to grow. If theinitiation reaction takes place very rapidly in comparison with thegrowth reaction, very narrow molecular weight distributions areobtainable.

If a monomer having at least one ethylenically unsaturated group is notcapable of anionic polymerization, the procedure can be modified so thatpolymerization can be continued by, for example, a free radical orcationic methods. The possibilities for transferring from anionicpolymerization to growth mechanisms otherwise initiated are described,for example, in P. Rempp, E. Franta and J. E. Herz, Advances in PolymerScience 1988, pages 164-168.

However, it is also possible to connect blocks produced by anionicpolymerization to blocks which are obtainable only by polycondensationor polyaddition of monomeric building blocks (e.g. polyesters orpolyurethanes), by, for example, adding an anionically produced block,provided with a suitable functional terminal group, during apolycondensation (e.g. R. N. Young, R. P. Quirk and L. J. Fetters,Advances in Polymer Science, 56 (1984), 70). Wilhelm, M. et al.,Macromolecules 24 (1991), 1033, relates to, for example,polystyrene/polyethylene oxide two-block polymers. The preparation ofblock polymers by free radical polymerization with the aid of functionalinitiators or macroinitiators is described, for example, in B. Riess, G.Hurtrez and P. Bahadur in “Encyclopedia of Polymer Science andEngineering”, Vol. 2, 327-330, Wiley & Sons (1985).

U.S. Pat. No. 4,581,429, U.S. Pat. No. 5,322,912 and U.S. Pat. No.5,412,047 describe the preparation of block polymers via pseudo-livingfree radical polymers. This procedure is essentially applicable to allmonomers having at least one ethylenically unsaturated group and alsopermits the preparation of block polymers having a particularly uniformmolecular weight.

Of course, block polymers can be converted into other block polymers bysubsequent chemical reactions (for example, polymer-analogousreactions). Polymer Preprints (Am. Chem. Soc. Div. Polym. Chem.) 29(1988), 425-426, relates to, for example, two-block polymers which areobtainable by anionic sequential polymerization of first an alkyl esterof methacrylic acid and then glycidyl methacrylate and subsequentconversion of the oxiranyl groups into β-hydroxysulfonate groups.Macromolecules 26 (1993), 7339-7352, discloses, for example, thepreparation of polystyrene/polyacrylic acid two-block polymers byhydrolysis of polystyrene/poly(tert-butyl acrylate) two-block polymers.

In general, separately preprepared polymeric blocks can be linked whenthey have suitable functional terminal groups. Thus, EP-A 665 240describes, for example, polymethacrylate/polymethacrylic acid two-blockpolymers in which the two blocks are linked by a constitutional unit

If the various blocks of constitutional units which may link with oneanother, and initiator radicals, if required moderator radicals andterminating radicals, are neglected, it is possible to represent blockpolymers in a simple manner by placing the basic unit of the respectiveblock in square brackets and indicating by means of a number attachedoutside the square brackets the number of times the respective blockcontains the basic unit linked to itself. The preparation of the blockas a function of time can be reproduced by the sequence of squarebrackets.

It is generally known that amphiphilic block polymers dissolved in waterare capable of acting like classical surfactants if the hydrophobic andhydrophilic blocks contained are of suitable length, and it is for thisreason that they are also referred to as polymeric surfactants and arerecommended, inter alia, as dispersants for stabilizing aqueous polymerdispersions.

EP-A 665 240 recommends, for example, two-block polymers

[alkyl methacrylate]_(r) [methacrylic acid]_(s)

where r and s are each from 4 to 20, as dispersants in aqueous polymerdispersions. This recommendation is repeated by the applicant of EP-A665 240 in its company brochure 17-1784-92/5D. A comparablerecommendation is given in Proc. Int. Org. Coat. Sci. Technol. 20th(1994), 511-518.

Polymer Preprints (Am. Chem. Soc. Div. Polym. Chem.) 29 (1988), 425-426,recommends two-block polymers

[alkyl methacrylate/α-methylstyrene]_(p) [sulfonated glycidylmethacrylate]_(q)

where p is <20, as dispersant in aqueous polymer dispersions.

Macromolecules 24 (1991), 5922-5925, shows that, when an aqueoussolution of abovementioned two-block polymers is added to an aqueousstandard polymer dispersion (a mixture of 20 g of ethyl acrylate, 80 gof water, 100 mg of K₂S₂O₈ and 0.02 g of sodium oleyl tauride sulfonateis initially taken in a polymerization vessel with stirring (150revolutions per minute) and freed from oxygen by means of a stream ofnitrogen; the mixture is then heated to 80° C. and is polymerized for 30minutes while maintaining this temperature and continuing the stirring;the resulting mean polymer particle diameter is 80 nm (photoncorrelation spectroscopy), the two-block polymer is rapidly attracted tothe surface of the disperse polymer particles (increase in thehydrodynamic radius of the disperse polymer particles, determined bymeans of photon correlation spectroscopy) and permits restabilization ofthe disperse phase.

According to the invention, however, what is particularly preferred isthe addition of those amphiphilic substances which, when added as a 10⁻³molar micellar aqueous solution to the abovementioned aqueous standardpolymer dispersion (in a total amount of 3% by weight, based ondispersed polymer, of amphiphilic substance) at 20° C. and 1 atm withmoderate stirring (5 revolutions per minute) in the course of 15,preferably 30, minutes, particularly preferably in the course of 1 hour,very particularly preferably in the course of 5, more preferably 10,especially 20, hours, do not result in any increase in the hydrodynamicradius of the dispersed standard polymer particles, i.e. according tothe invention the dispersing effect of the amphiphilic substance to beadded is only of minor importance.

Advantageous substances to be added according to the invention areaccordingly those in whose micellar aqueous solutions in a concentrationrange from 10⁻⁴ to 10⁻³ mol/l (where obtainable) at 20° C. and 1 atm theaverage residence time of a unimer within a micelle is at least 15,preferably at least 30, minutes, particularly preferably at least 1hour, very particularly preferably at least 5, more preferably at least10, especially at least 20, hours (free radical aqueous emulsionpolymerizations are generally carried out at from 0 to 100° C.; ofcourse, the temperature influences the kinetics of micellar systems;however, the temperature effect in the case of the abovementionedsystems is not so pronounced that a standardization according to theinvention to 20° C. would no longer be justified).

The average residence time of a unimer in a micelle can be determined ina manner known per se, for example by marking an amphiphilic unimer typein two different ways, preparing from the differently marked unimers twoseparate aqueous micellar solutions which contain the micelles marked incorrespondingly different ways, mixing these solutions with one anotherand then observing the time-dependent establishment of an aqueousmicellar solution of micelles having mixed marking.

A simple possible method of marking comprises covalently marking thehydrophobic group with two fluorophores which differ from one another,the two fluorophores being chosen so that, when they approach to adistance of ≦10 nm, the decay in the fluorescence of one fluorophore(donor) due to nonradiant energy transfer (Forster transfer) to theother fluorophore (acceptor) is shortened.

If a micellar solution whose unimers are exclusively donor-marked in theabovementioned manner and an aqueous micellar solution whose unimers areexclusively acceptor-marked in the abovementioned manner are preparedand the two solutions are then mixed, unimer exchange gives micelleswhich contain both donor-marked and acceptor-marked unimers and do so ata distance of <10 nm, since the hydrophobic groups point into theinterior of the micelles. The time-dependent change in the decay offluorescence of the donor fluorophore as a function of this exchangegives the average residence time in a manner known per se.

The end value of the time-dependent decay in fluorescence can bedetermined by first dissolving a mixture of donor-marked andacceptor-marked unimers in an organic solvent, separating off thesolvent, for example by distillation and producing the required aqueousmicellar solution from the resulting powder, the micelles of whichsolution then contain from the outset both donor-marked andacceptor-marked unimers (cf. Förster, Zeitschrift fur Naturforschung, A4(1949), 321; The synthesis of polymers bearing terminal fluorescent andfluorescence-quenching groups in Macromol. Chem. 191 (1990), 3069;Langmuir 1993, 1741-1748; Macromol. Chem. Macromol. Symp. 58 (1992),195-199; Collect. Czech Chem. Commun. 58 (1993), 2362; Macromolecules 25(1992), 461-469).

Another possible method of marking is isotopic marking, which results inunimers or micelles having different masses. The measured quantity usedin this case may be, for example, one which is dependent on the mass. Ameasured quantity of this type which is suitable for the micellaraqueous solutions of the amphiphilic substances to be added according tothe invention is in particular the migration rate in a centrifugal fieldof the analytical ultracentrifuge (AUC). If, in addition to unimers, themicellar aqueous solution contains, for example, two micellar types, theAUC sedimentation (for example, W. Machtle in S. E. Harding et al. (Ed.)Analytical Ultracentrifugation in Biochemistry and Polymer Science,Royal Society of Chemistry, Cambridge, England (1992), chapter 10, 147;Langmuir 9 (1993), 1741-1748 and Macromol. Chem. Macromol. Symp. 58(1992), 195-199) has two schlieren peaks corresponding to the twomicellar types, the area under the respective peak being proportional tothe absolute amount in each case. If the two micellar types of differentmass now fuse as a function of time to give one micellar type of averagemass, the two abovementioned schlieren peaks disappear as a function oftime and a third schlieren peak appears. The time characteristic givesthe desired average residence time in a manner known per se.

Furthermore, the AUC sedimentation run is capable of showing thefraction dissolved in unimer form in the micellar aqueous solutions ofthe amphiphilic substances to be added according to the invention.

According to the invention, it is preferable to add amphiphilicsubstances in whose micellar aqueous solutions in the concentrationrange from 10⁻⁴ to 10⁻³ mol/l (if accessible) at 20° C. and 1 atm thefraction dissolved in unimer form is ≦5 , preferably ≦3 , particularlypreferably ≦1 or ≦0.5, % by weight, based on the total amount ofamphiphilic substance contained in dissolved form.

Furthermore, the AUC sedimentation diffusion equilibrium method (cf. W.Mächtle in Makromol. Chem., Rapid Commun. 13 (1992), 555-563, permitteddetermination of the weight average molecular weight of the micellaraggregates contained under a schlieren peak.

Starting from the amphiphilic block polymers recommended in the priorart as dispersants for aqueous polymer dispersions, amphiphilicsubstances to be added according to the invention are obtainable byincreasing the number of basic units contained in the hydrophobic blocks(the length of the hydrophobic block) and at the same time establishingthe length of the hydrophobic blocks so that they permit the dissolutionof the micellar aggregate.

According to the invention, the associated loss of surfactant activityis essentially insignificant since the object according to the inventionis the use as a quasi-aqueous seed polymer dispersion.

The dominant effect of the length of the hydrophobic block is evident,for example, from the c.m.c. of aqueous solutions of two-block polymersof the general formula

[Styrene]_(m) [Na salt of acrylic acid]_(n)

If n is chosen as 1000 and at the same time m is increased from 6 to110, the c.m.c. in aqueous solution at 20° C. and 1 atm decreases by afactor of about 300.

If, on the other hand, m is chosen as 110 and n is varied in the rangefrom 300 to 1400, the c.m.c. in aqueous solution at 20° C. and 1 atmdoes not even change by a factor of about 2. It is striking that, for aconstant value of n=1000, the c.m.c. decreases particularly dramaticallyon changing m from 6 to 30, while at m≧40 the c.m.c. scarcely decreaseswith increasing m. This finding is of interest in that our owninvestigations have shown that the transition to micellar aqueoussolutions of frozen micelles takes place at m≧30.

Essentially similar results are obtained for two- or three-blockpolymers

[styrene]_(m)′[methacrylic acid]_(n)′,

[styrene]_(m)″[ethylene glycol]_(n)″,

[methyl methacrylate]_(m)′″ [Na salt of acrylic acid]_(n)′″,

[methyl methacrylate]_(m)″″ [ethylene glycol]_(n)″″,

[methacrylic acid]_(n)′″″ [styrene]_(m)′″″ [methacrylic acid]_(n)′″″

and [styrene]_(m)* [acrylic acid]_(n)*.

The group consisting of the amphiphilic substances to be added accordingto the invention therefore includes in particular two- and three-blockpolymers of the general formulae I, I′ and II

]A]_(a) [B]_(b) (I), [B]_(b) [A]_(a) (I′), and [A]_(a′) [B]_(b) [A]_(a′)or [A]_(a′) [B]_(b)[A]_(a″) (II),

where

B is a basic unit selected from the group consisting of styrene,methylstyrene, chlorostyrene, vinyl esters of C₁-C₈-alkanecarboxylicacids, esters of an α,β-monoethylenically unsaturated carboxylic acid of3 to 6 carbon atoms and a C₁- to C₈-alkanol, butadiene and ethylene and

A(A′) is a basic unit selected from the group consisting ofα,β-monoethylenically unsaturated mono- and dicarboxylic acids of 3 to 6carbon atoms, 2-acrylamido-2-methylpropanesulfonic acid, styrenesulfonicacid, vinylsulfonic acid and the alkali metal (in particular Na and K)and ammonium salts of the abovementioned acids, N-vinylpyrrolidone,vinyl alcohol, ethylene glycol and propylene glycol.

Of course, both the A(A′) and the B block may be copolymers having acorresponding degree of polymerization and comprising the monomers ofthe respective group (this statement also relates to the groups singledout below as being preferred).

B is preferably a basic unit selected from the group consisting ofstyrene, methylstyrene, chlorostyrene, vinyl acetate, vinylpropionate,methyl acrylate, ethyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylateand methyl methacrylate.

B is particularly preferably a basic unit selected from the groupconsisting of styrene, methyl methacrylate, n-butyl acrylate and2-ethylhexyl acrylate. The B block is very particularly preferablycomposed of styrene and/or methyl methacrylate.

A(A′) is preferably a basic unit selected from the group consisting ofacrylic acid, methacrylic acid, vinylsulfonic acid,2-acrylamido-2-methylpropanesulfonic acid and the Na, K and NH₄ salts ofthese acids.

The A(A′) block is particularly preferably composed of acrylic acid,methacrylic acid and/or the K, Na and NH₄ salts thereof.

b is, as a rule, an integer ≦30, preferably ≧35, particularly preferably≧40, very particularly preferably ≧45 or ≧50 (in the case of blockpolymers which are nonuniform with regard to the molecular weight, thestatements made here are based on the number average values of thecoefficients b, a, a′ and a″). Advantageous two- and three-blockpolymers of the general formulae I and II are also those in which b ≧75or b ≧100. Usually, b is ≦1000 or ≦800, in general ≦600 and frequently≦400. The range from b ≧30 to 40 is also of particular interest sinceits applications permits the preparation of finely divided aqueouspolymer dispersions. This applies in particular when the basic units forthe blocks A(A′) and B are selected from the particularly preferredgroups. The block B is advantageously chosen, with regard to its monomercomposition and its length, so that, as an independent polymer, it has aglass transition temperature Tg of ≧20° C., preferably ≧40° C.,particularly preferably ≧60° C., very particularly preferably ≧80° C.and even more preferably ≧100° C. Tg means the quasi-static glasstransition temperature measured by means of DSC (differential scanningcalorimetry, 20° C./min., midpoint) according to DIN 53765. The upperlimits for Tg in the case of high molecular weights and homopolymericblocks B are shown in, for example, Table 8 in Ullmann's Encyclopedia ofIndustrial Chemistry, VCH, Weinheim (1992), Vol. A21, page 169. It isnot possible to make a generally valid statement with regard to thelength of the blocks A which is required for solubilizing the blocks Band with regard to the question concerning the extent to which anaqueous micellar solution is obtainable by direct dissolution in wateror only by indirect dissolution, but the answer can be obtained in thespecific case by means of a few preliminary experiments. As a rule,micellar solubilization, required according to the invention in aqueoussolution is possible in the case of the two-block polymers I when a isat least 40% or at least 50% of b. As a rule, a is at least 75% of b andfrequently a is ≧b. a values of up to 2500 or more are possible.

In the case of three-block polymers II, the micellar solubilization,required according to the invention, in aqueous solution are possible ingeneral when a′ and a″, independently of one another, are at least 20%or at least 25% of b, respectively. As a rule, they are at least 40% ofb and frequently a′ and a″ are ≧b/2 or ≧b. a′ and a″, values of up to2000 or more are possible.

However, if the ratio of a/b or (a′+a″)/b is too great, the micellaraqueous solution generally converts into a molecular aqueous solution.

A direct micellar aqueous solution of the two- and three-block polymersof the general formulae I and II is frequently possible when the basicunit A is the salt of a monoethylenically unsaturated organic acid. If,on the other hand, the basic unit A is the free acid, it is as a rulenecessary to use the method of indirect dissolution (initiallydissolution in a mixture of water and an organic solvent, the latteradvantageously being chosen so that the block B, as an independentpolymer, would be soluble in this solvent; thereafter, gradualdisplacement of the organic solvent by means of dialysis and/or additionof water in combination with removal of the solvent by distillation).Frequently, direct micellar dissolution of the free acid in alkalinewater is also possible.

Two- and three-block polymers of the general formulae I and II, whosehydrophilic blocks A have a polyelectrolyte character (i.e. not nonionicbut dissociating into a polyion and opposite ions in an aqueous medium),are generally preferred as amphiphilic substances to be used accordingto the invention.

It is noteworthy that the micellar aqueous solutions of the amphiphilicsubstances to be added according to the invention solubilize monomers tobe polymerized by the free radical aqueous emulsion polymerizationmethod and having at least one vinyl group more selectively than themicellar aqueous solutions of the classical surfactants, i.e. when theyare used for controlled free radical aqueous emulsion polymerizations,it is advisable to match with one another the hydrophobic micellar coreand the monomers to be polymerized, i.e. to choose them so that they arechemically similar to one another. Accordingly, fluorine-containinghydrophobic blocks B are less preferable according to the invention.

Some individual examples of block polymers I and II which are suitableas amphiphilic substances to be added according to the invention areshown below. A d following in brackets indicates that the method ofdirect dissolution has been adopted for obtaining the not less than 10⁻⁴molar micellar aqueous solution, while an i following in a correspondingmanner indicates indirect solubilization (as a rule starting fromdioxane/water mixtures):

[styrene]₂₆₀ [methacrylic acid]₃₈₅ (i)

[styrene]₃₈₁ [methacrylic acid]₃₂₀ (i)

[styrene]₂₃₃ [methacrylic acid]₂₄₀ (i)

[styrene]₃₃₃ [methacrylic acid]₂₆₈ (i)

[styrene]₃₁₇ [methacrylic acid]₂₅₆ (i)

[styrene]₂₁₁ [methacrylic acid]₂₀₉ (i)

[styrene]₂₃₀ [methacrylic acid]₂₄₄ (i)

[styrene]₂₀₂ [methacrylic acid]₃₁₄ (i)

[styrene]₃₂₇ [methacrylic acid]₂₆₇ (i)

[styrene]₃₄₆ [methacrylic acid]₄₄₂ (i)

[methacrylic acid]₅₂ [styrene]-₇3 [methacrylic acid]₅₂ (i)

[methacrylic acid]₉₃ [styrene]₂₁₂ [methacrylic acid]₉₃ (i)

[methacrylic acid]₁₃₂ [styrene]₃₀₁ [methacrylic acid]₁₃₂ (i)

[methacrylic acid]₁₇₄ [styrene]₅₂₉ [methacrylic acid]₁₇₄ (i)

[methacrylic acid]₁₃₂ [styrene]₆₀₄ [methacrylic acid]₁₃₂ (i)

[styrene]₄₀ [sodium acrylate]₈₂ (d)

[styrene]₄₀ [sodium acrylate]₁₈₀ (d)

[styrene]₄₀ [sodium acrylate]₅₂₀ (d)

[styrene]₄₀ [sodium acrylate]₂₄₀₀ (d)

[styrene]₈₆ [sodium acrylate]₁₀₀ (d)

[styrene]₈₆ [sodium acrylate]₁₉₀ (d)

[styrene]₈₆ [sodium acrylate]₃₉₀ (d)

[styrene]₈₆ [sodium acrylate]₁₀₀ (d)

[styrene]₁₁₀ [sodium acrylate]₃₈₀ (d)

[styrene]₁₁₀ [sodium acrylate]₂₄₀₀ (d)

[methyl methacrylate]₃₉ [sodium acrylate]₇₉ (d)

[methyl methacrylate]₈₂ [sodium acrylate]₈₂ (d)

[methyl methacrylate]₈₀ [sodium acrylate]₈₀ (d)

[methyl methacrylate]₇₉ [sodium acrylate]₈₆ (d)

[methyl methacrylate]₃₅ [sodium acrylate]₁₀₅ (d)

[methyl methacrylate]₃₆ [sodium acrylate]₃₃ (d)

[methyl methacrylate]₃₉ [sodium acrylate]₃₉ (d)

[methyl methacrylate]₃₁ [sodium acrylate]₁₁₀ (d)

[methyl methacrylate]₄₀ [sodium acrylate]₈₀ (d)

[methyl methacrylate]₄₉ [sodium acrylate]₈₈ (d)

[methyl methacrylate]₇₀ [sodium acrylate]₇₅ (d)

[methyl methacrylate]₆₂₀ [sodium acrylate]₆₂₀ (i, Tetrahydrofuran).

As stated above, the block polymers I and II which are suitable asamphiphilic substances to be added according to the invention areobtainable, for example, by the anionic sequential polymerization method(living polymers). Instead of the unsaturated organic acids, such asacrylic acid and methacrylic acid, frequently their tert-butyl estersare first copolymerized and then converted into the acid form byhydrolysis. In a similar manner, in some cases the anhydrides are alsocopolymerized instead of the acids and are subsequently hydrolyzed.

However, the preparation of the block polymers I and II is preferablycarried out by the free radical sequential polymerization method, i.e.via pseudo-living free radical polymers, as described in U.S. Pat. No.4,581,429, U.S. Pat. No. 5,322,912 and U.S. Pat. No. 5,412,047. The keyto the procedure disclosed in these publications is to carry out thefree radical polymerization in the presence of stable free radicals,e.g. N-oxyl radicals, which leads to polymers which can be reactivatedto give free radical polymers and can thus continue to grow after theaddition of further monomers.

Examples of such suitable N-oxyl free radicals are:

2,2,6,6-tetramethyl-1-pyrrolidinyloxy (TEMPO),

4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy (4-oxo-TEMPO),

4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy,

2,2,5,5-tetramethyl-1-pyrrolidinyloxy,

3-carboxy-2,2,5,5-tetramethylpyrrolidinyloxy and

di-tert-butyl nitroxide.

2,6-Diphenyl-2,6-dimethyl-1-piperidinyloxy and2,5-diphenyl-2,5-dimethyl-1-pyrrolidinyloxy may also be used. Mixturesof the abovementioned compounds may of course also be employed.

The sequential free radical polymerization is carried out as a rule atelevated temperatures, advantageously at from 100 to 180° C., preferablyfrom 110 to 175° C., in particular from 130 to 160° C. It may be carriedout either in the absence of a solvent or in solution (or by the freeradical aqueous emulsion polymerization method). The free radicalpolymerization is advantageously initiated by conventional free radicalinitiators having a short half-life, making it possible to obtainparticularly low nonuniformities N (=ratio of weight average to numberaverage molecular weight=M_(W)/M_(n)). Suitable conventional freeracical initiators of this type include 2,4-dimethyl-2,5-dibenzylperoxyhexane, tert-butyl peroxybenzoate, di-tert-butyldiperoxyphthalate, methyl ethyl ketone peroxide, dicumyl peroxide,tert-butyl peroxycrotonate, 2,2-bis-tert-butyl(peroxybutane),tert-butylperoxy isopropyl carbonate,2,5-dimethyl-2,5-bis(benzoylperoxy)hexane, tert-butyl peracetate,2,4-pentadiene peroxide, di-tert butyl peroxide,2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne,2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, preferably tert-butylperoxy-2-ethyl hexanoate, tert-butyl peroxypivalate, tert-amylperoxy-2-ethylhexanoate, azobisalkyl nitriles, such asazobisisobutylronitrile, and diaryl peroxides, such as dibenzoylperoxide, and mixtures of the abovementioned compounds. Particularlysuitable conventional free radical initiators to be used concomitantlyare those having a half-life of about 1 hour at from 60 to 90° C. Themolar ratio of N-oxyl free radical to conventional free radicalinitiator should be from 0.5 to 5, preferably from 0.8 to 4. Values from1 to 3 or from 1 to 2, in general from 1 to 1.5, frequently from 1 to1.2 and often from 1 to 1.1, can thus regularly be achieved for N. Blockpolymers having N values in the abovementioned ranges are generallysuitable according to the invention.

Instead of starting from stable N-oxyl free radicals, it is also posibleto start from compounds (for example the alkoxyamines of U.S. Pat. No.4,581,429) which decompose, for example, under the action of heat withformation of a stable N-oxyl free radical and of a free radicalpolymerization initiator. Furthermore, such N-oxyl free radicals can beproduced in situ from suitable compounds having an NO function (cf.column 5, line 60 et seq. of U.S. Pat. No. 4,581,429).

According to the invention, block polymers having a very uniformmolecular weight are advantageous in that, in their micellar aqueoussolutions, the micelles generally have an essentially uniform size,their use for controlled free radical aqueous emulsion polymerizationfacilitating the preparation of essentially monodisperse (with regard tothe diameter of the disperse polymer particles) aqueous polymerdispersions.

Micellar aqueous solutions of the amphiphilic substances to be addedaccording to the invention and having a broad micelle size distributionmay be either used as such for controlled free radical aqueous emulsionpolymerization (as a rule, aqueous polymer dispersions having a broaderpolymer particle diameter distribution then result) or (owing to theirlong life) fractionated beforehand according to their size in theanalytical ultracentrifuge and can then be used according to theinvention as portions monodisperse in this manner.

Another possible method of fractionation is flow-field-flowfractionation, which sorts according to the micellar hydrodynamicdiameter. A description of this fractionation method is given in, forexample, Anal. Chem. 64 (1992), 790-798. A measure of the quality of thefractionation is the ratio M_(W)/M_(n). The micellar weight averagemolecular weight is given, for example, by classical light scattering,and the micellar number average molecular weight is obtainable, forexample, by membrane osmometry.

The block polymers obtainable by sequential free radical polymerizationaccording to U.S. Pat. No. 4,581,429, U.S. Pat. No. 5,322,912 and U.S.Pat. No. 5,412,047 have, as a rule, a terminal oxyamine group. Forvarious reasons, elimination of the oxyamine groups may be desirable. Incolumn 6, line 54 et seq., U.S. Pat. No. 4,581,429 offers variouspossible elimination methods of this type. Of particular interestaccording to the invention are those which lead to an —H, a hydroxylgroup or an ethylenically unsaturated terminal group. The last methodmakes it possible to obtain ethylenically unsaturated block polymers(macromers) which are of interest according to the invention in thatthey are chemically bonded to the dispersed polymer particles in thecourse of the free radical aqueous emulsion polymerization. Verygenerally, the preparation of block polymers to be used according to theinvention is carried out by initiated polymerization, preferably so thatany hydrophobic initiator or moderator radical terminates thehydrophilic block. The hydrophobic block may of course also beterminated in this manner.

The novel process for the preparation of an aqueous polymer dispersionby polymerization of monomers having at least one vinyl group by thefree radical aqueous emulsion polymerization method can be carried outin a simple manner by mixing a micellar aqueous solution of theamphiphilic substance to be added according to the invention, ifrequired further aqueous dispersing medium, the monomers to bepolymerized and the free radical polymerization initiator with oneanother in a polymerization vessel, heating the mixture to thepolymerization temperature while stirring and maintaining thepolymerization, while stirring, until the desired polymerizationconversion is obtained.

The polymerization temperature is adapted to the characteristics of thedispersing medium and to the initiator system used and is usually from20 to 100° C. It is often from 50 to 95° C. and frequently from 70 to90° C.

The free radical aqueous emulsion polymerization is usually carried outat atmospheric pressure (1 atm). However, it may also be carried outunder superatmospheric pressure, in particular when monomers which aregaseous at atmospheric pressure are used. In a corresponding manner,polymerization temperatures above 100° C. are also possible (e.g. up to130° C.). The abovementioned conditions are also typical for the otherfree radical aqueous emulsion polymerization processes discussed in thispublication. In the case of the emulsion polymerization methoddescribed, in which the total amount of the polymerization batch isinitially taken in the polymerization vessel, the size of the resultingpolymer particles is determined essentially by the type and amount ofthe amphiphilic substance contained in the batch and to be addedaccording to the invention. With an increasing amount of the amphiphilicsubstance contained in the batch, smaller polymer particles areobtained, and vice versa. Doubling the amount of the relevantamphiphilic substance contained in the polymerization batch is usuallyassociated with a doubling of the number of polymer particles formed andpresent as the disperse phase.

However, the disadvantage of the emulsion polymerization methoddescribed (total batch initially taken), is that it is suitable only forthe preparation of aqueous polymer dispersions having a relatively lowpolymer content.

In the polymerization method in which the total batch is initiallytaken, for example, problems with regard to the technicalcontrollability of the removal of the heat of reaction of the exothermicpolymerization reaction occur at the polymer contents relevant inpractice (as a rule >25% by weight).

On an industrial scale, the free radical aqueous emulsion polymerizationis therefore carried out, as a rule, by the feed method, i.e. thepredominant amount (as a rule from 50 to 100% by weight) of the monomersto be polymerized is added to the polymerization vessel at the rate ofprogress of the polymerization of the monomers already present in thepolymerization vessel (polymerization conversion as a rule ≧80 or ≧90 or≧95 mol%). In order to control the particle size of the resultingaqueous polymer dispersion, according to the invention a micellar aqeoussolution of the amphiphilic substance to be added according to theinvention is initially taken in the polymerization vessel in the feedmethod. The ratio of initially taken micelles (and their type) tomonomers to be polymerized essentially determines the size of thepolymer particles in the resulting aqueous polymer dispersion. Thesmaller the initially taken micelles and the greater their number, thesmaller are the resulting polymer particles with a given amount ofmonomer. If the initially taken number of micelles is increasedseveral-fold, as a rule the number of polymer particles formed ismultiplied in a corresponding manner in the novel procedure. In the feedmethod, preferably not more than up to 20% by weight of the monomers tobe polymerized are initially additionally taken in the polymerizationvessel. After the beginning of the free radical aqueous emulsionpolymerization, the remaining monomers are fed in during the novel feedmethod so that, at any time during the feed, the polymerizationconversion of all monomers added beforehand to the polymerization vesselis at least 80, preferably at least 90, mol %.

The manner in which the free radical initiator system is added to thepolymerization vessel in the course of the novel free radical aqueousemulsion polymerization in the feed method tends to be of minorimportance. The initiator system may be either initially taken in itsentirety in the polymerization vessel or added continuously or stepwiseat the rate at which it is consumed in the course of the novel feedmethod. Specifically, this depends, in the manner known per se to aperson skilled in the art, both on the chemical nature of the initiatorsystem and on the polymerization temperature.

If, in the course of the novel feed method, the amphiphilic substance tobe added according to the invention is introduced into thepolymerization vessel also or only during the monomer feed (i.e.initially taken mixture does not comprise the total amount of saidamphiphilic substance), this generally effects a controlled broadeningof the size distribution of the resulting polymer particles. Here too,the amphiphilic substance to be added according to the invention ispreferably introduced in the form of a prepared micellar aqueoussolution. For example, the feed methods of DE-A 42 13 969, DE-A 42 13968, DE-A 42 13 967, DE-A 42 13 964 and DE-A 42 13 965 may be applied inan appropriately adapted manner in order to prepare correspondinglyhighly concentrated aqueous polymer dispersions. For this purpose, theaqueous starting dispersions to be used in the abovementioned laid-openapplications should be replaced in a simple manner by correspondingnovel micellar aqueous solutions.

Suitable free radical polymerization initiators for the novel processare all those which are capable of initiating a free radical emulsionpolymerization. These may be both peroxides, for example alkali metalperoxodisulfates, and azo compounds. For polymerizations at lowtemperatures, combined systems which are composed of at least oneorganic reducing agent and at least one peroxide and/or hydroperoxide,e.g. tert-butyl hydroperoxide and the sodium salt ofhydroxymethanesulfinic acid or hydrogen peroxide and ascorbic acid (asan electrolyte-free redox initiator system), are preferably used, veryparticularly preferably combined systems which additionally contain asmall amount of a metal compound which is soluble in the polymerizationmedium and whose metallic component may occur in a plurality of valencystates, e.g. ascorbic acid/iron(II) sulfate/hydrogen peroxide, thesodium salt of hydroxymethanesulfinic acid, sodium sulfite, sodiumbisulfite or sodium disulfite also frequently being used instead ofascorbic acid, and tert-butyl hydroperoxide or an alkali metalperoxodisulfate and/or ammonium peroxodisulfate also frequently beingused instead of hydrogen peroxide. Instead of a water-soluble iron(II)salt, a combination of water-soluble Fe/V salts is frequently used.

As a rule, the amount of free radical initiator systems used is from 0.1to 2% by weight, based on the total amount of the monomers to bepolymerized.

It is noteworthy that the novel process does not necessarily require thepresence of additional dispersants in order to obtain an aqueous polymerdispersion of satisfactory stability. Such aqueous polymer dispersionswhich are free of further dispersants and are obtainable by the novelprocess are advantageous in that they have particularly little tendencyto foam and a comparatively high surface tension.

However, conventional dispersants may of course be present in the novelprocess, for further stabilization of the disperse phase of the polymerparticles produced. To maintain control over the novel process, however,it is preferable, when additional dispersants are present, to ensurethat the amounts are such that the c.m.c. of these additionaldispersants are not exceeded. It is frequently advisable to effectrestabilization by the addition of conventional dispersants after theend of the novel free radical aqueous emulsion polymerization.

Examples of such conventional dispersants are the classical surfactants.Dowfax® 2A1 from Dow Chemical Company, ethoxylated mono-, di- andtrialkylphenols (degree of ethoxylation: from 3 to 50, alkyl radical: C₄to C₉), ethoxylated fatty alcohols (degree of ethoxylation: from 3 to50, alkyl radical: C₈ to C₃₆) and alkali metal and ammonium salts ofalkylsulfates (alkyl radical: C₈ to C₁₂), of sulfuric half-esters ofethoxylated alcohols (degree of ethoxylation: from 4 to 30, alkylradical: C₁₂ to C₁₈) and ethoxylated alkyl phenols (degree ofethoxylation: from 3 to 50, alkyl radical: C₄ to C₉), of alkanesulfonicacids (alkyl radical: C₁₂ to C₁₈) and of alkylarylsulfonic acids (alkylradical: C₉ to C₁₈) may be mentioned by way of example. Further suitablesurfactants are described in Houben-Weyl, Methoden der organischenChemie, Volume XIV/1, Makromolekulare Stoffe, Georg-Thieme Verlag,Stuttgart, 1961, pages 192 to 208. However, conventional protectivecolloids, such as polyvinyl alcohol, polyvinylpyrrolidone or amphiphilicblock polymers having short hydrophobic blocks may also be used forcostabilization, instead of, or as a mixture with, classicalsurfactants. As a rule, the amount of conventional dispersants presentdoes not exceed 3 or 2% by weight, based on the monomers to bepolymerized.

Monomers which are capable of free radical polymerization and aresuitable for the novel process are in particular monoethylenicallyunsaturated monomers, such as olefins, e.g. ethylene, vinylaromaticmonomers, such as styrene, a-methylstyrene, o-chlorostyrene orvinyltoluenes, vinyl and vinylidene halides, such as vinyl andvinylidene chloride, esters of vinyl alcohol and monocarboxylic acids of1 to 18 carbon atoms, such as vinyl acetate, vinyl propionate,vinyl-n-butyrate, vinyl laurate and vinyl stearate, esters ofα,β-monoethylenically unsaturated mono- and dicarboxylic acids of,preferably, 3 to 6 carbon atoms, in particular acrylic acid, methacrylicacid, maleic acid, fumaric acid and itaconic acid, with alkanols of, ingeneral, 1 to 12, preferably 1 to 8, in particular 1 to 4, carbon atoms,in particular methyl, ethyl, n-butyl, iso-butyl and 2-ethylhexylacrylate and methacrylate, dimethyl maleate or n-butyl maleate, nitrilesof α,β-monoethylenically unsaturated carboxylic acids, such asacrylonitrile, and conjugated C₄-C₈-dienes, such as 1,3-butadiene andisoprene. As a rule, the stated monomers are the main monomers whichtogether usually account for more than 50% by weight, based on the totalamount of the monomers to be polymerized by the novel free radicalaqueous emulsion polymerization method. Monomers which, when polymerizedby themselves, usually give homopolymers which have a high watersolubility are usually copolymerized only as modifying monomers inamounts of less than 50, as a rule, from 0.5 to 20, preferably from 1 to10, % by weight, based on the total amount of the monomers to bepolymerized.

Examples of such monomers are α,β-monoethylenically unsaturated mono-and dicarboxylic acids of 3 to 6 carbon atoms and their amides, e.g.acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconicacid, acrylamide and methacrylamide, and vinylsulfonic acid and itswater-soluble salts and N-vinylpyrrolidone. Monomers which usuallyincrease the internal strength of the films of the aqueous polymerdispersion are as a rule likewise copolymerized only in minor amounts,in general from 0.5 to 10% by weight, based on the total amount ofmonomers to be polymerized. Usually, such monomers have an epoxy,hydroxyl, N-methylol or carbonyl group or at least two nonconjugatedethylenically unsaturated double bonds. Examples of these areN-alkylolamides of α,β-monoethylenically unsaturated carboxylic acids of3 to 10 carbon atoms and their esters with alcohols of 1 to 4 carbonatoms, among which N-methylolacrylamide and N-methylolmethacrylamide arevery particularly preferred, silanized monomers, such asy-methacryloyloxypropylsilane or vinyl trimethoxysilane, monomers havingtwo vinyl radicals, monomers having two vinylidene radicals and monomershaving two alkenyl radicals. The diesters of dihydric alcohols withα,β-monoethylenically unsaturated monocarboxylic acids are particularlysuitable, among which acrylic and methacrylic acid are once againpreferably used. Examples of such monomers having two nonconjugatedethylenically unsaturated double bonds are alkylene glycol diacrylatesand dimethacrylates, such as ethylene glycol diacrylate, 1,3-butyleneglycol diacrylate, 1,4-butylene glycol diacrylate and propylene glycoldiacrylate, divinylbenzene, vinyl methacrylate, vinyl acrylate, allylmethacrylate, allyl acrylate, diallyl maleate, diallyl fumarate,methylenebisacrylamide, cyclopentadienyl acrylate or triallyl cyanurate.Also of particular importance in this context are thehydroxy-C₁-C₈-alkyl esters of methacrylic and acrylic acid, such ashydroxyethyl, hydroxy-n-propyl or hydroxy-n-butyl acrylate andmethacrylate, and compounds such ass diacetoneacrylamide and acetylacetoxyethyl acrylate and methacrylate. In addition to monomers havingunsaturated double bonds, molecular weight regulators, such astert-dodecyl mercaptan and 3-mercaptopropyltrimethoxysilane, may becopolymerized in minor amounts, usually in an amount of from 0.01 to 2%by weight, based on the monomers to be polymerized. Such substances arepreferably added to the polymerization zone as a mixture with themonomers to be polymerized.

In particular, monomer mixtures which can be subjected to free radicalaqueous emulsion polymerization by the novel process in a controlledmanner to give aqueous polymer dispersions are those which are composedof

from 70 to 100% by weight of esters of acrylic and/or methacrylic acidwith alkanols of 1 to 12 carbon atoms and/or styrene or

from 70 to 100% by weight of vinyl chloride and/or vinylidene chlorideor

from 70 to 100% by weight of styrene and/or butadiene or

from 40 to 100% by weight of vinyl acetate, vinyl propionate and/orethylene

the acrylate mixtures comprising in particular the following monomercompositions which consist of:

from 70 to 99% by weight of at least one ester of acrylic and/ormethacrylic acid with alkanols of 1 to 8 carbon atoms,

from 1 to 5% by weight of at least one monomer selected from the groupconsisting of acrylic acid, methacrylic acid and the K^(⊕), Na^(⊕) andammonium salts of these acids and

from 0 to 25% by weight of vinyl acetate, styrene or a mixture thereof.

Finally, it should once again be noted that the attractiveness of theamphiphilic substances to be added according to the invention asassistants for the free radical aqueous emulsion polymerization does notlie in an effect whereby the surface tension of the aqueous medium isreduced and the disperse phase thus stabilized. Rather, it is itscontrolling influence on the resulting number of dispersed polymerparticles which is striking. The extent of its effect whereby itsimultaneously stabilizes the disperse phase of the polymer particlesproduced, in spite of having little effect on the surface tension of thewater, is surprising. It is noteworthy that the aqueous polymerdispersions resulting according to the invention are suitable both asbinders (e.g. paper coating slips, interior coating materials, fibremats) and as adhesives or additives in mineral, e.g. cement-containing,binders.

Of course, the novel process is usually carried out under an inert gasand with stirring. As a rule, the amount of amphiphilic substance addedaccording to the invention is from 0.1 to 15, preferably from 0.5 to 6,% by weight, based on the monomers to be polymerized. If required, thenumber of micelles in the aqueous solution can be calculated from themicellar molecular weight (for example, determinable from the course ofsedimentation in the analytical ultracentrifuge or by classical lightscattering) and the sample weight, and the desired number of polymerparticles thus established in advance.

Finally, it should be noted that the solids volume contentration ofaqueous polymer dispersions obtainable according to the invention may befrom 10 to 70 or from 25 to 70 or from 35 to 70 or from 45 to 70, % byvolume. The resulting number average polymer particle diameter may befrom 10 to 2000 nm, from 50 to 1500 nm, from 100 to 1000 nm, from 200 to750 nm or from 300 to 500 nm.

It should also be noted that aqueous polymer dispersions obtainableaccording to the invention can be dried in a simple manner withoutadditional assistants to give redispersible polymer powders (forexample, by freeze drying or spray drying). This applies in particularwhen the glass transition temperature of the added amphiphilic substanceis ≧60° C., preferably ≧70° C., particularly preferably ≧80° C. and veryparticularly preferably ≧90° C. or ≧100° C. Usually, it does not exceed250° C.

EXAMPLES

1) Preparation of amphiphilic two-block polymers BP1 to BP6 by anionicpolymerization (cf. Macromolecules 24 (1991), 4997)

a) Preparation of the initiator solution (initiator:

diphenylhexyllithium)

Tetrahydrofuran (from Riedel de Haen, chromasolv) was dried by addingn-butyllithium (the end point was indicated by means of styrene;remaining, unhydrolyzed n-butyllithium initiates anionic polymerizationof the styrene; the intense red color of the resulting styrenemacroanions acts as an indicator) and degased by repeated application ofreduced pressure. After the end of the drying, 90 ml of tetrahydrofuranwere distilled off under reduced pressure and freed from final traces ofO₂ by freezing, applying a vacuum and thawing, this procedure beingcarried out three times.

6 ml of n-butyllithium (as a 1.6 molar solution in hexane from Acros)was first dissolved in the 90 ml of anhydrous and oxygen-freetetrahydrofuran under an inert gas atmosphere to give a homogeneoussolution. An equivalent amount (1.8 ml=10 mmol) of diphenylethylene(from Fluka, purum) was then added (dark red diphenylhexyllithium wasimmediately formed). The oxygen-free and anhydrous mixture was thenshaken at 20° C. for a further 2 days with the exclusion of light. Thecontent of initiator formed was finally determined by a testpolymerization of methyl methacrylate and subsequent determination ofthe molecular weight of the resulting polymer by means of gel permeationchromatography.

b) Preparation of the monomers

I. Methyl methacrylate

30 g of methyl methacrylate were degased by repeated brief applicationof reduced pressure. For deactivation of protic impurities,triethylaluminum (as 15% strength by weight solution in hexane, fromFluka, purum) was then added until a marked yellow color occurred (thiswas the case after the addition of 2.4 ml of the hexane solution).Thereafter, reduced pressure was again applied briefly, and finally ⅔ ofpure monomer were distilled off under reduced pressure and frozen, andreduced pressure was again applied.

II. tert-Butyl acrylate

A corresponding amount of tert-butyl acrylate was subjected to aprocedure similar to that for the 30 g of methyl methacrylate. Finally,however, the tert-butyl acrylate was diluted to 3 times its volume withoxygen-free and anhydrous tetrahydrofuran. This tert-butylacrylate/tetrahydrofuran solution was stored in the frozen state untiluse.

c) Anionic polymerization (oxygen-free and anhydrous)

1.5 g of anhydrous LiCl (improves the initiator quality by decomposingdimers, etc.) were initially taken in a polymerization vessel. 400 ml oftetrahydrofuran were then added with condensation, 66.35 g of theprepared initiator solution were added and the resulting mixture wascooled to just above the freezing point of tetrahydrofuran (−108.5° C.)by means of liquid nitrogen. 23 g of tert-butyl acrylate (as theprepared solution in tetrahydrofuran) were then added dropwise in thecourse of 2 minutes with vigorous stirring. The liquid mixture was keptat −78° C. for 15 minutes with stirring. 18 g of the prepared methylmethacrylate were then added dropwise with continued stirring in thecourse of 1 minute, and the reaction mixture was stirred for a further15 minutes at −78° C. The anionic polymerization was then stopped byadding 5 ml of degased methanol to which a few drops of acetic acid hadbeen added. The reaction mixture was then warmed up to room temperature,a part of the solvent was stripped off under reduced pressure and theblock polymer formed was then precipitated in 1.5 1 of a methanol/watermixture (in the volume ratio 2:1) and dried overnight at 50° C. in adrying oven under reduced pressure.

A 1:1 molar ratio of the starting monomers used was verified in theresulting block polymer by means of ¹H-NMR (200 MHz) in a CDC1₃solution. The relative number average molecular weight M_(n) of theblock polymer formed was determined as 8758 by means of gel permeationchromatography. The relative weight average molecular weight M_(W) was9957. The most frequent relative molecular weight was 9678. An averageblock polymer of

[tert-butyl acrylate]₃₉ [methyl methacrylate]₃₉ corresponds to thismolecular weight.

d) Selective hydrolysis of the tert-butyl acrylate

20 g of the block polymer were dissolved in 100 ml of dioxane andstirred with 11 ml of 37% strength by weight aqueous HCl for 4 hours at80° C. After cooling to 25° C., the resulting polymer was precipitatedin n-heptane (GR, from Merck), filtered off, washed with n-heptane andthen dried for 1 week at 80° C. in a drying oven under reduced pressure(in all other cases, isolation was effected by freeze drying fromdioxane).

The selectivity and completeness of the hydrolysis was confirmed bymeans of ¹H-NMR (200 MHz) in a CD₃OD solution.

A hydrophilic block polymer BP1

[acrylic acid]₃₉ [methyl methacrylate]₃₉ was thus obtained.

The following hydrophilic block polymers BP2 to BP7 were prepared in acorresponding manner:

BP2: [acrylic acid]₇₉ [methyl methacrylate]₃₉

BP3: [acrylic acid]₃₃ [methyl methacrylate]₃₆

BP4: [acrylic acid]₁₀₅ [methyl methacrylate]₃₅

BP5: [acrylic acid]₃₀ [methyl methacrylate]₂₈

BP6: [acrylic acid]₃₀ [methyl methacrylate]₂₅

BP7: [acrylic acid]₂₆ [methyl methacrylate]₁₈

The block polymer BP8 was additionally acquired from TH. Goldschmidt AG(=experimental product MA1007 from TH. Goldschmidt AG):

BP8: [methyl methacrylate]₁₀ [methacrylic acid]₈.

2) Determination of the surface tension of micellar aqueous solutions ofthe sodium salts of the two-block copolymers from 1)

The determinations of the surface tension were carried out using a ringtensiometer from Lauda (TElc) at 20° C. and 1 atm. The aqueous solutionsinvestigated were directly obtainable. The results obtained are shown inTable 1 below.

TABLE 1 Concentration Sodium salts of (mol/l) σ (mN/m) BP8 1.96 · 10⁻⁶65.52 BP8 2.94 · 10⁻⁶ 62.44 BP8 5.88 · 10⁻⁶ 57.42 BP8 9.79 · 10⁻⁶ 53 BP81.47 · 10⁻⁵ 50.81 BP8 2.44 · 10⁻⁵ 48.41 BP8 3.90 · 10⁻⁵ 46.32 BP8 6.11 ·10⁻⁵ 44.82 BP8 9.64 · 10⁻⁵ 43.61 BP8 1.51 · 10⁻⁴ 42.69 BP8 2.36 · 10⁻⁴41.77 BP8 3.66 · 10⁻⁴ 40.91 BP8 5.60 · 10⁻⁴ 40.13 BP8 8.40 · 10⁻⁴ 39.53BP8 1.23 · 10⁻³ 39.01 BP8 1.74 · 10⁻³ 38.55 BP8 2.35 · 10⁻³ 38.29 BP83.02 · 10⁻³ 37.59 BP1 4.90 · 10⁻⁷ 71.56 BP1 7.35 · 10⁻⁷ 71.56 BP1 1.47 ·10⁻⁶ 71.56 BP1 2.45 · 10⁻⁶ 71.51 BP1 3.67 · 10⁻⁶ 71.48 BP1 6.10 · 10⁻⁶71.43 BP1 9.74 · 10⁻⁶ 71.39 BP1 1.53 · 10⁻⁵ 71.33 BP1 2.41 · 10⁻⁵ 71.27BP1 3.77 · 10⁻⁵ 71.16 BP1 5.90 · 10⁻⁵ 71.11 BP1 9.15 · 10⁻⁵ 71.08 BP11.40 · 10⁻⁴ 71.06 BP1 2.10 · 10⁻⁴ 71.02 BP1 3.07 · 10⁻⁴ 71.01 BP1 4.34 ·10⁻⁴ 71 BP1 5.87 · 10⁻⁴ 70.88 BP1 7.54 · 10⁻⁴ 70.53 BP2  5.2 · 10⁻⁴ 63.1BP3  1.3 · 10⁻³ 64.5 BP4  4.5 · 10⁻³ 63.6 BP5   1 · 10⁻⁵ 57.1 BP6  1.1 ·10⁻³ 48.4 BP7  1.4 · 10⁻³ 46.7

A graphical plot of the results is shown in FIG. 1. It shows that thesodium salts of BP8, BP7, BP6 and BP5 are not amphiphilic substancesaccording to the invention. In particular, the sodium salt of BP8behaves like a classical surfactant. The ammonium salts give resultssimilar to those for the sodium salts.

3) Preparation of aqueous polymer dispersions D1 to D13 by the freeradical aqueous emulsion polymerization method with the addition ofNH^(⊕) ₄ salts of BP1 and BP8 as amphiphilic two-block polymers

An ammoniacal solution of the block polymer was initially taken in thepolymerization vessel, with or without the addition of water.Thereafter, the initially taken solution was heated to 90° C. and 30% byweight of a feed 2 were then added all at once. Two minutes later, theremaining amount of feed 2 was fed continuously (in the course of 2hours) into the polymerization vessel and, beginning at the same timeand spatially separately therefrom, a feed 1 was added continuously (inthe course of 1 hour and 30 minutes) while maintaining the temperatureof 90° C. After the end of the feeds, the polymerization mixture wasstirred for a further hour at 90° C. and then cooled to roomtemperature. Essentially mono-disperse aqueous polymer dispersions wereobtained, and they were characterized in the analytical ultracentrifugeby determining the d₅₀ values (50% by weight of the polymer has aparticle diameter≧d₅₀ and 50% by weight of the polymer has a particlediameter≦d₅₀.

Tables 2 and 3 below show the compositions of initially taken solution,feed 1 and feed 2 and the d₅₀ values of the resulting aqueous polymerdispersions.

TABLE 2 Initi- ally taken Solu- solu- tion tion of D1 D2 D3 D4 D5 BP10.03 g 0.15 g 0.3 g 1.5 g 3 g NH₃ 0.003 g 0.015 g 0.03 g 0.15 g 0.3 gWa- 1.47 g 7.335 g 14.67 g 28.35 g 56.7 g ter Wa- 54 g 48 g 41 g 25 g —ter Feed Me- 15 g 15 g 15 g 15 g 15 g 1 thyl meth- acry- late n- 15 g 15g 15 g 15 g 15 g But- yl acry- late Feed Sod- 0.15 g 0.15 g 0.15 g 0.15g 0.15 g 2 ium pero- xodi- sul- fate Wa- 15 g 15 g 15 g 15 g 15 g terd50 112 84 58 38 32 (nm)

TABLE 3 Initi- ally taken sol- Solution ution of D6 D7 D8 D9 D10 D11 D12D13 BP8 0.1 0.5 2 5 10 15 20 25 25% 3.2 3.2 6.4 6.4 6.4 6.4 6.4 6.4strength by weight aqueous NH₃ solution Feed Water 177 178 181 188 200211 223 235 1 Methyl 50 50 50 50 50 50 50 50 meth- acrylate n-Butyl 5050 50 50 50 50 50 50 acrylate Feed Water 60 60 60 60 60 60 60 60 2Sodium 2 2 2 2 2 2 2 2 peroxo- disul- fate d50 157 129 103 86 40 31 2626 (nm)

For a predetermined amount of monomer and essentially completepolymerization conversion, the number of polymer particles formed isinversely proportional to the cube of the particle diameter.

FIGS. 2 and 3 show a graphical plot of (1/d³ ₅₀)·10⁷ [nm-⁻³] for the d₅₀values shown in Tables 2 and 3, as a function of the amount of BP1 orBP8 added to the polymerization.

While an essentially linear relationship is obtained in the case of thenovel amphiphilic substance, FIG. 3 shows a virtually constant number ofresulting polymer particles in the range of small added amounts of BP8.This is due to the fact that, although the initially taken number ofmicelles increases with an increasing amount of added BP8, said micellesare not quantitatively initiated but a part of them dissolve again, inthe course of polymer particle formation, in the other part to stabilizethe polymer particle growth therein (if the polymerizations were notcarried out by the feed procedure but all components were intially takenin the mixture and then heated to the reaction temperature, essentiallyidentical results would be obtained).

4) Investigations of aqueous solutions of the ammonium salt of BP1 andaqueous polymer dispersions containing this salt, using an analyticalultracentrifuge

Aqueous solutions which each contained 6, 3, 2 or 1 g of BP1 per literand 0.4 g of a 25% strength by weight aqueous NH₃ solution per gram ofBP1 were investigated. Furthermore, the aqueous solutions investigatedcontained 0.1 mol of NH₄Cl per liter, in order to facilitate thedissociation of polyanion and ammonium ions in the centrifugal field.The temperature of the investigation was 20° C. In the sedimentation run(40000 revolutions per minute), only a monomodal schlieren peak occurredin all cases. The sedimentation rate was 7.2 Svedberg in all cases. Themagnitude of the sedimentation rate showed that the units separating outmust be aggregates of BP1 unimers and not BP1 unimers, indicating themicellar character of the aqueous solutions investigated. Since theschlieren peak comprised at least 98% of the amount of BP1 drawn in,these sedimentation runs showed that all micellar aqueous solutionscontained less than 2% of the sample weight of BP1 in dissolved form.Investigtions by membrane osmometry confirmed this result (permeablecellulose acetate membrane of a relative molecular weight <10000) inthat the osmotic pressure building up as a function of time did not passthrough a maximum.

The apparent weight average molecular weight of the units migrating inthe centrifugal field was determined as a function of the solutionconcentration from the exponential concentration curves ofsedimentation-diffusion equilibrium runs carried out with theabovementioned aqueous solutions (absorbence as a function of themicelle radius, λ=236 nm, 6000 revolutions per minute, 90 hours). Theplot of the reciprocal values as a function of concentration gave arelative M_(W) of the migrating unit of 600000±25% on linearextrapolation to the concentration 0 (cf. FIG. 4).

In sedimentation runs carried out in a corresponding manner with D5(contains the largest amount of BP1) in a very wide range of dilutions,no free BP1 dissolved in unimer form was detectable. When D5 wasdissolved in excess tetrahydrofuran and this solution was covered with alayer of pure tetrahydrofuran, the resulting Schlieren peak had asedimentation rate of 0.9 Svedberg, indicating a relative molecularweight of the sedimenting unit of roughly 7000, which essentiallycorresponds to BP1 dissolved in unimer form.

When additional ammonium salt of BP1 was added to D5 and the mixture wasleft alone with gentle stirring at 20° C. for 40 hours, the Schlierenpeak of the unimeric BP1 contained at least 99% of the subsequentlyadded amount of BP1 even after this time, i.e. the amphiphilic blockcopolymer was not attracted to the dispersed polymer particles and itsmicellar solution was retained.

5) Experiment to determine the critical micellar formation concentrationof the ammonium salt of BP1 in water at 20° C. and 1 atm by means ofclassical light scattering

While FIG. 1 indicates a c.m.c. of just less than 10^(−4.5) mol/l forthe sodium salt of BP8, the concentration-dependent surface tensionmeasurement was not suitable for the c.m.c. determination in the case ofthe sodium salt of BP1. The determination thereof was thereforeattempted by means of classical light scattering.

For this purpose, a stock solution of 1 g of BP1, 0.4 g of 25% strengthby weight NH₃ solution and 300 g of water was prepared and was dilutedby means of 0.1 molar aqueous NH₄Cl solution. The difference in thescattering intensity (in the forward direction in the angle segment from6 to 7°) between solvent and solution was determined as a function ofthe solution concentration of BP1, with a low-angle laserlight-scattering photometer KMX-6 from Chromatix (USA), using an He/Nelaser (kλ=633 nm).

FIG. 5 shows a plot of the inverse values of these scattering intensitydifferences as a function of the concentration.

The concentration-dependent curve shows (no deflection to higher inversevalues with increasing dilution) that the c.m.c. is not reached even ata dilution of 10⁻⁷ mol/l. The extrapolation value determined in FIG. 5by linear extrapolation to c=0 reflects a weight average relativemolecular weight of 650000±10% and confirms the results obtained in theanalytical ultracentrifuge.

6) Forster transfer measurements of the exchange of unimers betweenmicelles

In order to investigate the exchange of [acrylic acid]_(i) [methylmethacrylate]_(j) two-block polymers in their micellar aqueous solutionsat 20° C., two-block polymers of comparable composition were covalentlymarked on the one hand with the donor naphthaline and on the other handwith the acceptor pyrene:

[acrylic acid]₈₂ [methyl methacrylate₈₂-pyrene and

[acrylic acid]₈₆ [methyl methacrylate₇₉-naphthaline.

Aqueous solutions of the sodium salts of the two two-block polymers werethen prepared (dissolve block polymer in NaOH-containing water), saidsolutions containing, in addition to water, the following per liter ofsolution:

200 mg of two-block polymer (calculated completely as acid) and 42.4 or44.8 mg of NaOH.

Equal volumes of the two separately prepared aqueous solutions weremixed with one another in a cell of a flash fluorescence spectrometer(Edinburgh Instruments, FL 900 CDT, method of measurement: time-resolvedsingle photon counting (TCSPC)). Time-resolved fluorescence decays(excitation wavelength: 290 nm (hydrogen (0.46 bar)-filled flash lamp),pulse duration: 1 ns, pulses/sec: 4000, detection wavelength(naphthaline): 340 nm) of the naphthaline donor showed no change within20 hours. The same applied on heating at 80° C. for 2 hours.

7) Examples of further novel aqueous free radical emulsionpolymerizations

The following two-block copolymers were prepared according to Example1):

a) [acrylic acid]₃₀ [methyl methacrylate]₃₅

b) [acrylic acid]₈₀ [methyl methacrylate]₄₀

c) [methyl methacrylate]₄₉ [acrylic acid]88

d) [methyl methacrylate]₇₀ [acrylic acid]₇₅

e) [acrylic acid]₆₂₀ [methyl methacrylate]₆₂₀

The ammonium salts of the two-block copolymers a) to d) were used asfollows for the free radical aqueous polymerization:

The monomers (as a mixture of 50% by weight of n-butyl acrylate and 50%by weight of methyl methacrylate), water, 0.5% by weight (based on themonomers) of sodium peroxodisulfate and X pphm (based on the weight ofthe monomers) of the abovementioned ammonium salts were mixed with oneanother, heated to the polymerization temperature of 90° C. whilestirring and polymerized at this temperature to a conversion of >99% byweight (solids content chosen: 30% by weight). In all cases, aqueouspolymer dispersions of sufficient stability were obtained.

The Xpphm used were:

1 pphm two-block copolymer a);

1 pphm two-block copolymer b);

2 pphm two-block copolymer b);

1 pphm two-block copolymer c);

2 pphm two-block copolymer c);

1 pphm two-block copolymer d);

2 pphm two-block copolymer d).

The light transmittance (indirect measure of the polymer particle size)of the aqueous polymer dispersions (path length: 2 cm, aqueous polymerdispersion diluted to a solids content of 0.01% by weight, 25° C.,standardized to water (LD value=100), transmission of white light) was≧91 in all cases.

100 mg of the two-block polymer e) were dissolved in tetrahydrofuran.Thereafter, an equivalent aqueous sodium hydroxide solution was addeddropwise and the aqueous solution of the sodium salt of the two-blockcopolymer e) was produced by evaporating off the tetrahydrofuran. Thesolution was diluted to 20 g with water. Thereafter, 1 g of methylmethacrylate and 5 mg of Na₂P₂O₈ were added, the resulting mixture washeated to the polymerization temperature of 80° C. while stirring andpolymerization was effected at this temperature to a conversion of >95%by weight. An aqueous polymer dispersion of satisfactory stability wasobtained.

All polymerizations were reproducible in a satisfactory manner.

We claim:
 1. A process for preparing an aqueous polymer dispersion bypolymerizing monomers having at least one vinyl group by the freeradical aqueous emulsion polymerization method, in which an amphiphilicsubstance is added to the polymerization vessel before or during thepolymerization or both, wherein i) 1 l of water at 20° C. and 1 atm iscapable of taking up at least 10⁻⁴ mol of the amphiphilic substance inmicellar solution; ii) the critical micelle formation concentration ofthe amphiphilic substance at 20° C. and 1 atm in water is <10⁻⁶ mol/l;and iii) the surface tension of an aqueous or micellar solution or bothof the amphiphilic substance in a molar concentration range(0<C_(m)≦10⁻⁴)mol/l at 20° C. and 1 atm does not fall below 60 mN/m;wherein the amphiphilic substance is a two-block polymer of the formula(I) or (I′): [A]_(a)[B]_(b)  (I) [B]_(b)[A]_(a)  (I′) wherein: [B]_(b)is a copolymer block or a homopolymer block comprising monomers selectedfrom the group consisting of styrene, methyl styrene, chlorostyrene,vinyl esters of C₁-C₈ alkanecarboxylic acids, esters of anα,β-monoethylenically unsaturated carboxylic acid of 3 to 6 carbon atomsand a C₁-C₈-alkanol, butadiene and ethylene; [A]_(a) is a copolymerblock or a homopolymer block comprising monomers selected from the groupconsisting of α,β-monoethylenically unsaturated mono- and dicarboxylicacids of 3 to 6 carbon atoms, 2-acrylamido-2-methylpropanesulfonic acid,styrene sulfonic acid, vinylsulfonic acid and the alkali metal andammonium salts of the abovementioned acids, N-vinylpyrrolidone, vinylalcohol, ethylene glycol and propylene glycol, b is an integer whichindicates the number of monomers contained in chemically bonded form inthe polymer block [B]_(b), b being ≧30, and a is an integer whosemagnitude is at least 40%, preferably at least 75%, of the magnitude ofb and which indicates the number of monomers contained in chemicallybonded form in the polymer block [A]_(a).
 2. The process as claimed inclaim 1, wherein the critical micelle formation concentration of theamphiphilic substance at 20° C. and 1 atm in water is ≦10^(−6.25) mol/l.3. The process as claimed in claim 1, wherein the critical micelleformation concentration of the amphiphilic substance at 20° C. and 1 atmin water is ≦10^(−6.5) mol/l.
 4. The process as claimed in claim 1,wherein the critical micelle formation concentration of the amphiphilicsubstance at 20° C. and 1 atm in water is ≦10^(−6.75) mol/l.
 5. Theprocess as claimed in claim 1, wherein the critical micelle formationconcentration of the amphiphilic substance at 20° C. and 1 atm in wateris ≦10⁻⁷ mol/l.
 6. The process as claimed in claim 1, wherein thecritical micelle formation concentration of the amphiphilic substance at20° C. and 1 atm in water is ≦10^(−7.25) mol/l.
 7. The process asclaimed in claim 1, wherein the critical micelle formation concentrationof the amphiphilic substance at 20° C. and 1 atm in water is ≦10^(−7.5)mol/l.
 8. The process as claimed in claim 1, wherein the surface tensionσ of an aqueous molecular or micellar solution of the amphiphilicsubstance in the molar concentration range (o<C_(M)≦10⁻⁴)mol/l at 20° C.and 1 atm does not fall below 62.5 mN/m.
 9. The process as claimed inclaim 1, wherein the surface tension σ of an aqueous molecular ormicellar solution of the amphiphilic substance in the molarconcentration range (0<C_(M)≦10⁻⁴)mol/l at 20° C. and 1 atm does notfall below 65 mN/rn.
 10. The process as claimed in claim 7, wherein thesurface tension σ of an aqueous molecular or micellar solution of theamphiphilic substance in the molar concentration range(0<C_(M)≦10⁻⁴)mol/l at 20° C. and 1 atm does not fall below 67.5 mN/r.11. The process as claimed in claim 1, wherein the surface tension σ ofan aqueous molecular or micellar solution of the amphiphilic substancein the molar concentration range (0<C_(M)≦10⁻⁴)mol/l at 20° C. and 1 atmdoes not fall below 70 mN/m.
 12. The process as claimed in claim 1,wherein the surface tension σ of an aqueous molecular or micellarsolution of the amphiphilic substance in the molar concentration range(0<C_(M)≦10⁻⁴)mol/l at 20° C. and 1 atm does not fall below 71.5 mN/m.13. The process as claimed in claim 1, wherein 1 l of water at 20° C.and 1 atm is capable of taking up at least 10⁻³ mol of the amphiphilicsubstance in micellar solution.
 14. The process as claimed in claim 1,wherein 1 l of water at 20° C. and 1 atm is capable of taking up atleast 10⁻² mol of the amphiphilic substance in micellar solution. 15.The process as claimed in claim 1, wherein 1 l of water at 20° C. and 1atm is capable of taking up at least 10⁻¹ mol of the amphiphilicsubstance in micellar solution.
 16. The process as claimed in claim 1,wherein 1 l of water at 20° C. and 1 atm is capable of taking up atleast 1 mole of the amphiphilic substance in micellar solution.
 17. Theprocess as claimed in claim 1, wherein the average residence time of aunimer within a micelle in a 10⁻⁵ molar micellar aqueous solution of theamphiphilic substance at 20° C. and 1 atm is at least 15 mi.
 18. Theprocess as claimed in claim 1, wherein the average residence time of aunimer within a micelle in a 10⁻⁵ molar micellar aqueous solution of theamphiphilic substance at 20° C. and 1 atm is at least 30 min.
 19. Theprocess as claimed in claim 1, wherein the average residence time of aunimer within a micelle in a 10⁻⁵ molar micellar aqueous solution of theamphiphilic substance at 20° C. and 1 atm is at least 1 hour.
 20. Theprocess as claimed in claim 1, wherein the average residence time of aunimer within a micelle in a 10⁻⁵ molar micellar aqueous solution of theamphiphilic substance at 20° C. and 1 atm is at least 10 hours.
 21. Theprocess as claimed in claim 1, wherein the average residence time of aunimer within a micelle in a 10⁻⁵ molar micellar aqueous solution of theamphiphilic substance at 20° C. and 1 atm is at least 20 hours.
 22. Theprocess as claimed in claim 1, wherein the amphiphilic substance isadded as a preformed micellar solution.
 23. The process as claimed inclaim 22, wherein the amphiphilic substance is added as a preformedaqueous micellar solution.
 24. The process as claimed in claim 23,wherein the preformed aqueous solution is a solution of frozen micelles.25. The process as claimed in claim 1, wherein [B]_(b) is a copolymerblock or a homopolymer block comprising monomers selected from the groupconsisting of styrene, methylstyrene, chlorostyrene, vinyl acetate,vinyl propionate, methyl acrylate, ethyl acrylate, n-butyl acrylate,2-ethylhexyl acrylate and methyl methacrylate.
 26. The process asclaimed in claim 25, wherein [B]_(b) is a copolymer block or ahomopolymer block comprising monomers selected from the group consistingof styrene, methyl methacrylate, n-butyl acrylate and 2-ethylhexylacrylate.
 27. The process as claimed in claim 26, wherein [B]_(b) is acopolymer block or a homopolymer block comprising monomers selected fromthe group consisting of styrene and methyl methacrylate.
 28. The processas claimed in claim 1, wherein [A]_(a) is a copolymer block or ahomopolymer block comprising monomers selected from the group consistingof acrylic acid, methacrylic acid, vinylsulfonic acid,2-acrylamido-2-methylpropanesulfonic acid, and the Na^(⊕), K^(⊕), andNH^(⊕) ₄ salts thereof.
 29. The process as claimed in claim 1, wherein[A]_(a) is a copolymer block or a homopolymer block comprising monomersselected from the group consisting of acrylic acid, methacrylic acid,and the K^(⊕), Na^(⊕), and NH₄ ^(⊕) salts thereof.
 30. The process asclaimed in claim 1, wherein b is ≧35.
 31. The process as claimed inclaim 1, wherein b is ≧40.
 32. The process as claimed in claim 1,wherein b is ≧45.
 33. The process as claimed in claim 1, wherein b is≧50.
 34. The process as claimed in claim 1, wherein a is ≧b.
 35. Theprocess as claimed in claim 1, wherein the monomer mixture to bepolymerized is composed of a) from 70 to 100% by weight of esters ofacrylic or methacrylic acid or both with alkanols of 1 to 12 carbonatoms or styrene or both, or b) from 70 to 100% by weight of vinylchloride or vinylidene chloride or both, or c) from 70 to 100% by weightof styrene or butadiene, or both, or d). from 40 to 100% by weight ofvinyl acetate, vinyl propionate or ethylene or a combination thereof.36. The process as claimed in claim 1, wherein the polymerization iscarried out by the feed method.
 37. The process as claimed in claim 1,wherein the total amount of the amphiphilic substance to be added isinitially taken in the polymerization vessel.
 38. The process as claimedin claim 1, wherein at least some of the amphiphilic substance to beadded is not added until after the beginning of the polymerization. 39.An aqueous polymer dispersion obtained by a process as claimed in claim1.
 40. A two-block polymer of the formula I or I′ [A]_(a)[B]_(b)  (I)[B]_(b)[A]_(a)  (I′) wherein: [B]_(b) is a copolymer block or ahomopolymer block comprising monomers selected from the group consistingof methylstyrene, chlorostyrene, vinyl esters of C₁C₈-alkanecarboxylicacids, esters of an α,β-monoethylenically unsaturated carboxylic acid of3 to 6 carbon atoms and a C₁-C₈-alkanol, butadiene and ethylene, [A]_(a)is a copolymer block or a homopolymer block comprising monomers selectedfrom the group consisting of α,β-monoethylenically unsaturated mono- anddicarboxylic acids of 3 to 6 carbon atoms,2-acrylamido-2-methylpropanesulfonic acid, styrene sulfonic acid,vinylsulfonic acid, the alkali metal and ammonium salts of theabovementioned acids, N-vinylpyrrolidone, vinyl alcohol, ethylene ,lycoland propylene glycol, b is an integer which indicates the number ofmonomers contained in chemically bonded form in the polymer block (B)b,b being ≧30, and a is an integer which indicates the number of monomerscontained in chemically bonded form in the polymer block [A]_(a), themagnitude of a being at least 40% of the magnitude of b.
 41. Thetwo-block polymer as claimed in claim 40, wherein [B]_(b) is a copolymerblock or a homopolymer block comprising monomers selected from the groupconsisting of esters of acrylic and methacrylic acid withC₁-C₈-alkanols, and [A]_(a) is a copolymer block or a homopolymer blockcomprising monomers selected from the group consisting of acrylic acid,methacrylic acid and the alkali metal and ammonium salts of these acids.42. The two-block polymer as claimed in claim 40, wherein [A]_(a) or[B]_(b) is a homopolymer block.
 43. The two-block polymer as claimed inclaim 40, wherein [B]_(b) is polymethyl methacrylate.
 44. The two-blockpolymer as claimed in claim 40, wherein [A]_(a) is a homopolyrner blockcomprising acrylic acid, methacrylic acid, potassium acrylate, potassiummenthacrylate, sodium acrylate, sodium methacrylate, ammonium acrylateor ammonium methacrylate.
 45. The two-block polymer as claimed in claim44, where 30≦b≦40 and a is at least 75% of b.
 46. The two-block polymeras claimed in claim 40, obtainable by sequential anionic polymerization.47. A two-block polymer of the formula (I) or (I′): [A]^(a)[B]_(b)  (I)[B]_(b)[A]_(a)  (I′) where [B]_(b) is a copolymer block or a homopolymerblock comprising monomers selected from the group consisting of styrene,methylstyrene, chlorostyrene, vinyl esters of C₁-C₈-alkanecarboxylicacids, esters of an a, p-monoethylenically unsaturated carboxylic acidof 3 to 6 carbon atoms and a C₁-C₈-alkanol, butadiene and ethylene,[A]_(a) is a copolymer block or a homopolymer block comprising monomersselected from the group consisting of α,β-monoethylenically unsaturatedmono- and dicarboxylic acids of 3 to 6 carbon atoms,2-acrylamido-2-methylpropanesulfonic acid, styrene sulfonic acid,vinylsulfonic acid and the alkali metal and ammonium salts of theabovementioned acids, b is an integer which indicates the number ofmonomers contained in chemically bonded form in the polymer block[B]_(b), b being ≧30, and a is an integer whose magnitude is at least40% of the magnitude of b and which indicates the number of monomerscontained in chemically bonded form in the polymer block [A]_(a),obtainable by sequential free radical polymerization in the presence ofN-oxyl free radicals.
 48. A method of preparing an aqueous polymerdispersion, which comprises preparing said aqueous polymer dispersionwith the two-block polymer as claimed in claim
 40. 49. An aqueouspolymer dispersion containing an amphiphilic substance, wherein i) 1 lof water at 20° C. and 1 atm is capable of taking up at least 10⁻⁴ molof the amphiphilic substance in micellar solution; ii) the criticalmicelle formation concentration of the amphiphilic substance at 20° C.and 1 atm in water is <10⁻⁶ mol/l ;and iii) the surface tension a of anaqueous and/or micellar solution of the amphiphilic substance in themolar concentration range (0<C_(M)≦10⁻⁴) mol/l at 20° C. and 1 atm doesnot fall below 60 mN/m, and wherein the amphiphilic substance is atwo-block polymer of the formula (I) or (I′): [A]_(a)[B]_(b)  (I)[B]_(b)[A]_(a)  (I′) wherein: [B]_(b) is a copolymer block or ahomopolymer block comprising monomers selected from the group consistingof styrene, methylstyrene, chlorostyrenic, vinyl esters of C₁-C₈alkanecarboxylic acids, esters of an α,β-monoethylenically unsaturatedcarboxylic acid of 3 to 6 carbon atoms and a C₁-C₈-alkanol, butadieneand ethylene, [A]_(a) is a copolymer block or a homopolymer blockcomprising monomers selected from the group consisting ofα,β-monoethylenically unsaturated mono- and dicarboxylic acids of 3 to 6carbon atoms, 2-acrylamido-2-methylpropanesulfonic acid, styrenesulfonic acid, vinylsulfonic acid and the alkali metal and ammoniumsalts of the abovementioned acids, N-vinylpyrrolidone, vinyl alcohol,ethylene glycol and propylene glycol, b is an integer which indicatesthe number of monomers contained in chemically bonded form in thepolymer block [B]_(b), b being ≧30, and a is an integer whose magnitudeis at least 40%, preferably at least 75%, of the magnitude of b andwhich indicates the number of monomers contained in chemically bondedform in the polymer block [A]_(a).
 50. A polymer powder obtainable bydrying an aqueous polymer dispersion as claimed in claim
 49. 51. Apolymer powder obtainable by drying an aqueous polymer dispersion asclaimed in claim
 39. 52. A method of preparing an aqueous polymerdispersion, which comprises preparing said aqueous polymer dispersionwith the two-block polymer as claimed in claim 40.